High temperature pressure vessels such as boilers include tube headers which consists of heavy walled piping, generally about 20 inches in diameter and over three inches thick. An array of tubes may vary from approximately 3/4 of an inch through 31/4 inch inside diameter and are typically 0.180 inches thick. Tubes are seated within bores in the header and are welded to the header. A change or step in diameter is created between the inside diameter of the tube and the inside diameter of the bore, the bore usually having a larger diameter than the inside of the tube.
It is known that after a period of use, a metal oxide film forms on the interior surfaces of the boiler tubes and tube header. This oxide scale exhibits hardness qualities beyond 60 Rockwell "C". Extreme care must be taken not to work harden this scale beyond its existing state. Scale removal must not radically distort the surface conditions of the base material either.
It is important however, to remove the oxide scale so as to permit inspection of the base material particularly within the tube header bores. This is because the surface of the bores within the tube headers are susceptible to cracking and these cracks are totally covered and obscured by the oxide scale.
One ordinary method for removing the scale is a mechanical one which requires cutting the tubes at an upper surface of the header and boring out the remaining tube stub. Boring equipment and cylinder hones would then be used to finish out the tube bore. A problem with this approach is that after the tube is replaced, the thick walled header must be stress relieved per ASME code. A field stress relief of this type would take days to perform and tens of thousands of dollars of cost, depending upon the size and thickness of the header.
Other methods of removing scale, in addition to physical removal of the tube, include the use of abrasive materials such as a sandpaper, the use of chemicals such as acids, and the use of stone hones. Each of these techniques however, have shortcomings. Friction from sandpaper heats up the oxide thereby making it even harder. Acid requires much time and eats away at the base material of the header and tubes. As noted above, physical removal of the tubes is extremely time consuming and expensive. Stone hones may break within the tubes or bores and this would cause even greater problems in trying to retrieve the breakage from the inside of boiler tubes.